This invention generally relates to casks for transporting radioactive materials, and is specifically concerned with an improved cask assembly for forming a cask adapted to transport a maximum amount of radioactive material of a particular activity within a given weight limit.
Casks for transporting radioactive materials such as the waste products produced by nuclear power plant facilities are known in the prior art. The purpose of such casks is to ship radioactive wastes in as safe a manner as possible. Such casks may be used, for example, to ship high-level vitrified waste cannisters to a permanent waste isolation site or spent fuel rods to a reprocessing facility. At the present time, relatively few of such transportation casks have been manufactured and used since most of the spent fuel and other wastes generated by nuclear power plants are being stored at the reactor facilities themselves. However, the availability of such on-site storage space is steadily diminishing as an increasing amount of fuel assemblies and other wastes are loaded into the spent-fuel pools of these facilities. Additionally, the U.S. Department of Energy (D.O.E.) has been recently obligated, by way of the National Waste Policy Act of 1983, to move the spent-fuel assemblies from the on-site storage facilities of all nuclear power plants to a federally operated nuclear waste disposal facility starting in 1998.
While the transportation casks of the prior art are generally capable of safely transporting wastes such as spent fuel to a final destination, the applicant has observed that there is considerable room for improvement, particularly With respect to vehicle-drawn, Type B casks. Specifically, the applicant has observed that, in many instances, the structure of these casks do not lend themselves to an optimal loading of radioactive wastes. The resulting less-than-optimum loading necessitates a larger number of trips by the shipper in order to complete the transportation of a given amount of radioactive waste, thus increasing both the time and the cost of transport. However, before the problems associated with optimizing the amount of waste carried by a particular cask may be fully appreciated, some understanding of the constraints imposed by NRC regulations is necessary.
U.S. Department of Transportation (DOT) and state highway regulations limit the gross weight of the waste-carrying road vehicle to about 80,000 pounds for shipments without special permits. Since the typical tractor and trailer weighs approximately 30,000 pounds, the weight of a cask and its contents must not exceed approximately 50,000 pounds. These same regulations specify that the surface radiation of such cask be no greater than 200 millirems at any given point, and that the radiation emitted by the cask be no greater than ten millirems at a distance of two meters from the vehicle. Other DOT regulations require that the cask be capable of sustaining impact stresses of up to ten Gs in the longitudinal direction, five Gs in the lateral direction, and two Gs in the vertical direction without yielding the wastes. The end result of these regulations is that much of the 50,000 pounds must be expended in providing adequate shielding materials within the cask (which are usually formed from dense materials such as lead or depleted uranium), as well as a structurally strong outer shell that can withstand the designated impact stresses. The thicknesses of both the shielding material and the structural shell required to comply with federal regulations leaves only a relatively small amount of space in the center of the cask which can actually be used to contain and transport radioactive waste. To maximize the amount of carrying volume, the most effective shielding materials known are frequently integrated into the walls of the cask structure. Such materials include lead, depleted uranium, and tungsten. However, as these materials are of a very high density, the radius of the cask walls cannot be made too large, or the gross weight limitation of 50,000 pounds of the combination of cask and waste material will be exceeded. The end result of the foregoing constraints of structural strength, shielding effectiveness, and the density of the most effective known shielding materials renders the carrying space in such cask relatively small relative to the volume of the cask as a whole when high activity wastes such as spent fuel rods are being transported.
If the cost of transporting a particular amount of radioactive waste is to be minimized, then the use of the carrying space within the cask must be maximized, i.e., the space must be completely filled up with a waste having an activity which brings the surface radiation of the cask, as a whole, to just under the 200 millirem limit. If the carrying space within the cask is completely filled with a waste, but the resulting surface radiation of the cask is substantially below 200 millirems per hour, then the use of the cask is not being optimized. In such a case, a cask having thinner walls with less shielding materials and a larger cavity would be the optimum choice for the transportation of such a waste. If, on the other hand, only a small amount of the carrying volume may be filled with a particular kind of waste before the surface radiation of the case reaches 200 millirems, then the large ring of air-space between the waste and the shielding material results in a highly ineffective shielding geometry, wherein an excessively large weight of shielding material is being used to comply with the surface radiation limit of 200 millirems. In short, there is a single, optimum activity that every static-walled, prior art cask is matched to. Nuclear waste having an activity which is substantially below or above this optimum activity results in significant inefficiencies wherein the ratio of cask weight to waste weight is considerably higher than desired.
Clearly, what is needed is a cask capable of optimally adjusting both the type and the amount of shielding materials contained within its walls to the particular type and activity of the waste material being hauled. Ideally, such a cask should be capable of quickly and conveniently adjusting the type and thickness of the shield materials used in its walls which are difficult to fabricate and machine, such as depleted uranium or tungsten. Finally, such a cask should be relatively simple and inexpensive to fabricate, and some sort of means for easily opening and closing the cask to effect loading and unloading operations, as well as a mechanism for reliably venting, purging, and draining the interior of the cask regardless of the particular type and thickness of shielding used in the cask interior.